Marmaduke m. Possibly Old Irish máel 'bald, tonsured' + Old Irish Máedoc 'of Máedoc', the name of a 6th-7th C Irish saint.
Spellings like Marmaduke were virtually restricted to North Yorkshire, England before the 16th C; if the origin of the name really is Old Irish (as Withycombe s.n. Marmaduke suggests), then this is one of the rare instances of a name which was borrowed from Irish into English, rather than the other way around.